Conventionally, a screening test device generally called as a burn-in apparatus performs screening of defective products in such a manner that, after packaging IC chips obtained by dividing a semiconductor wafer, a current applying test of the packaged IC chips is performed in a thermal atmosphere at a predetermined temperature (e.g., 125° C.), and thus latent defects are exposed.
Since such a conventional device needs a large constant-temperature device and has a large calorific value, the device needs to operate in a room separated from other production lines. Accordingly, it is desired to perform a burn-in test in a wafer level before becoming a chip because of: troubles such as transport of a wafer, attachment to and detachment from a device; wasteful packaging costs incurred by the detection of defective products after packaging; and a demand for a so-called bear chip quality-assured to mount the chip as it is without packaging the chip.
A burn-in apparatus for meeting these demands needs to keep a wafer at a uniform temperature, when a thermal load is applied to a semiconductor wafer. There has been proposed a wafer level burn-in apparatus having a temperature control function to keep a semiconductor wafer at a predetermined target temperature by providing heaters on both front and back sides of the wafer.
However, in the temperature control of the conventional method, a wafer surface is subjected to heating and cooling collectively. Accordingly, when an electric load is applied to the wafer, the electric load is not applied to any defective one of devices formed on the wafer. Therefore, variation in heat generation occurs due to the application of the electric load to the wafer and thus the temperature of the wafer is not uniform on the surface of the wafer. However, nothing has been done about it.
The conventional wafer level burn-in will be described in detail with reference to FIGS. 9, 10, and 11.
FIG. 9 is a schematic diagram illustrating a conventional wafer level burn-in apparatus. FIG. 10 is a diagram illustrating the distribution of conforming devices in wafer level burn-in. FIG. 11 is a diagram illustrating temperature variation of devices in conventional wafer level burn-in.
In FIG. 9, a wafer 101 is supported in a wafer support tray 102, the wafer 101 is connected to a substrate 104 applying an electric load through a probe 103 capable of collectively contacting the wafer 101, and an electric load is applied to the wafer 101 by a tester 105 having functions of applying electric loads, generating electric signals and comparing signals. A temperature load is applied by controlling the temperature of a temperature regulation plate 106 to 125° C. using a heater 108 and refrigerant such as water and alcohol flowing through a refrigerant flowing passage 107, which are disposed in the temperature regulation plate 106. A temperature regulator 110 controls the temperature of the temperature regulation plate 106 by controlling a calorific value of the heater 108 and controlling a temperature and a flow rate of the refrigerant flowing through the refrigerant flowing passage 107, on the basis of the temperature measured by a temperature sensor 109. In actual wafer level burn-in, devices are heated from a room temperature to 125° C. by the heater 108, an electric load is then applied to the devices on a wafer by the tester 105, and the tester 105 confirms whether the devices formed on the wafer break down or not at predetermined time intervals. In the course of the operation confirmation, the electric load of the tester 105 is cut off and the devices are operated by applying an electrical signal for the operation confirmation to the devices. Outputs from the devices are monitored by the tester 105 to confirm whether or not the devices break down due to the electric load and the temperature load.
FIG. 10 is a diagram illustrating the distribution of conforming products of the devices formed on the wafer. Hatched parts denote conforming devices and non-hatched parts denote defective devices. When an electric load is applied to the conforming products, the conforming products generate heat. However, since an electric load cannot be applied to the defective devices, the defective products do not generate heat. FIG. 11 is a graph illustrating temperature variation in wafer level burn-in between a conforming device 201 located in the vicinity of a part where conforming products are collected on the wafer in FIG. 10 and a conforming device 202 located in the vicinity of a part where defective products are collected. After the devices are heated to 125° C., an electric load is applied at a time T. Since the conforming device 201 located in the vicinity of the part where the conforming products are collected is at a high temperature, consumption of probe is accelerated. Since the conforming device 202 located in the vicinity of the part where the defective products are collected is at a low temperature, a temperature load set for the adjacent conforming devices cannot be applied.
An increase in diameter of wafer from 200 mm to 300 mm as well as an increase in calorific value at the time of applying the electric load on the wafer in conjunction with a decrease in size of IC chips or an increase in applied current result in decreasing in-plane uniformity of a wafer temperature. In the conventional wafer of 200 mm, a calorific value at the time of applying an electric load is about 400 W and a heat density is 12.74 kW/m2. However, in a wafer of 300 mm, it is considered that a calorific value is more than 3 kW, and in this case a heat density is 42.46 kW/m2. Simply assuming that the wafer support tray 102 in FIG. 9 is an aluminum plate with a thickness of 10 mm and with a thermal conductivity of 200 W/m·K, temperature difference caused by thermal conduction between two aluminum plates with a diameter of 200 mm and 300 mm is obtained. A portion with a center diameter of 20 mm in the aluminum plate with a diameter of 200 mm is controlled constantly to 125° C. When the generation of heat with a heat density of 12.74 kW/m2 occurs at the portion with the center diameter of 20 mm, a calorific value is 4.0 W. When all the heat is thermally conducted in a radius direction of the aluminum plate, the temperature of the aluminum plate on the circumference is 124.3° C. after stabilization. Similarly, when the generation of heat with a heat density of 42.46 kW/m2 occurs at a portion with a diameter of 20 mm in the aluminum plate with a diameter of 300 mm, a calorific value is 13.3 W and a temperature of the aluminum plate on the circumference is 122.1° C. Accordingly, it can be seen that the temperature difference increases more than the case of the aluminum plate with a diameter of 200 mm. In the actual wafer level burn-in, since the distribution of heat generation is different for each wafer according to the distribution of conforming devices formed on the wafer, it is difficult to control the temperature of the wafer to about 125° C. with the configuration of FIG. 9.